Particularly in the home heating area, countless furnaces of the gas and oil-fired type are used having flue pipes leading to chimneys or exhaust stack outlets. There has been difficulty in maximizing the efficiency of the flue pipe passageway cross section in order to maximize efficiency of the furnace and thus cut down the fuel requirements of the furnace to obtain best fuel efficiency and heat output. Often conventional procedure has been to merely insert a lateral flapper which in no way obstructs the flue pipe. Several flue pipe obstructing and adjusting dampers have also been suggested. These include various devices which operate substantially within flue pipe passagways by pivoting about axes perpendicular to the axis of the passageway. Such devices are disclosed in the following U.S. patents inter alia:
U.S. Pat. Nos. 1,599,063-Pawson; 2,441,398-D'Elia; 2,557,210-Viola et al; 2,735,385-De Ascentiis; 2,783,756-Crozier; 2,819,845-Ziph; 3,987,785-LeBeau; 4,046,318-Ripley.